1. Field of the Invention
The present invention generally relates to the manufacture of semiconductor devices and, more particularly, to high sensitivity in-situ monitoring and control of low temperature epitaxy processes for deposition of materials, especially silicon and silicon-germanium (SiGe) films.
2. Description of the Prior Art
Benefits of increased functionality and performance as well as manufacturing economy derived from increased integration density have encouraged developments in lithography and other semiconductor manufacturing processes to scale circuit elements such as transistors and memory cells to extremely small sizes. However, at such small sizes, relatively slight imperfections in the structures developed, such as variations in thickness of a silicon film, may become significant or even critical. Further, the composition of alloy films becomes of extreme importance since the conduction properties of alloys films may vary widely depending on relative proportions of materials in the alloy and even variations in proportions of materials with depth within the alloy film.
Unfortunately, process parameters such as temperature and reactant gas flows may not be easily monitored or accurately controlled and measurements are complicated by the dynamics of chemical reactions which are necessary to many semiconductor manufacturing processes. Therefore, at the current state of the art, deposition of films of certain desired properties is largely a matter of trial and error to develop suitable process parameters and the resulting film properties must be empirically verified through destructive testing which is slow and reduces process throughput and yield (since samples must be destroyed).
Specifically, for deposition of silicon and SiGe films at low temperatures, it is common practice to grow alloy films on a plurality of wafers simultaneously, since the growth of these films is typically very slow. When the process is complete, a sample wafer is removed from the batch and tested by, for example, secondary ion mass spectroscopy (SIMS) to determine the alloy composition profile over the thickness of the film. The SIMS process is very time-consuming and expensive and results may not be available for several days. Meanwhile, the remainder of wafers in the batch cannot be used in manufacturing and may not be usable at all if the SIMS testing does not confirm that the desired composition profile has been achieved.
Even for growth of silicon films, the rate of deposition during low temperature epitaxy processes varies exponentially with temperature and temperature drifts of only tens of degrees Centigrade can cause substantial variation in film thickness over the relatively long deposition times. Again, achievement of the desired film properties must be confirmed by destructive testing which is time consuming and expensive while delaying the utilization of similarly processed wafers or requiring them to be discarded or diverted to production of other devices for which the film may be appropriate.
The thermal mass of the reaction chamber (including the relatively large number of wafers being simultaneously processed) and potential temperature measurement inaccuracy (e.g. since the sensor(s) can not be ideally located) precludes temperature regulation to accuracies of less than the variation alluded to above and does not allow real-time monitoring and control of the progress of the deposition process.
One approach to non-destructive and/or in-situ determination of film thickness and properties has relied on optical reflection from a deposited surface since reflection, in turn depends on film thickness and refractive index of the deposited material. However, this technique has many shortcomings in that it relies on the additional growth of a test pattern comprising a layer of oxide and Si/SiGe to make viable measurements and such a structure may not be tolerated by the overall process requirements. Further, for real-time/in-situ measurement and/or control, the analyzing light must be passed through optical windows in the processing chamber which may be coated with the material being deposited; causing errors in the measurement which is of relatively low resolution even under ideal conditions. Moreover, chemical composition within the film cannot be accurately measured since thee optical interference relied upon inherently averages material properties over a thickness region.
The use of a residual gas analyzer (RGA) is known for monitoring the progress of growth of gallium arsenide (GaAs) films by setting the peak selector of the ion sensor to sense the introduced gas molecules, trimethyl gallium (TMG) and Arsine. The reaction byproduct, methane, is or would be seen for only one-half the growth cycle. This use of RGA, therefore, only qualitatively and indirectly observes the GaAs growth process to confirm that the process gases were, in fact, introduced into the chamber for a certain period of time. Further, this process does not and is not required to determine the ratio of Ga and As since the ratio is fixed at. 1:1 in a GaAs film, whereas the amount of each of the constituent materials in an alloy like SiGe can vary from substantially 0% to 100%.
Accordingly, it is seen that the present state of the art does not allow accurate monitoring or real-time control or compensation of film deposition processes or allow avoidance of time-consuming delays, loss of manufacturing yield and inefficiencies due to the cost of samples destructively tested. Moreover, the present state of the art does not provide any technique of developing particular desired alloy constituent concentrations other than by substantially trial and error methods. Since the constituent material concentration, such as the percentage of germanium in a SiGe alloy, controls all of the material and electrical properties of the alloy, it is critical to device fabrication methods to measure material concentration.